In theory, the best of the best, cast engines are as good as the cnc engines (the very odd one might even be better). As others have pointed out its for consistency between engines, that the change was made. Basically, buy ten cnc at random, have them fiched and set up properly and you might find a couple of 10th's between the lot. Do the same with ten cast and there will be a bigger spread I.e. Most will be average, one might be excellent and one a total dog. Test enough and you'll form a nice "bell shaped" distribution curve I.e most average, some a little better or worse and very few great or crap. Plus, the pool of good cast engines is only ever going to get smaller as they either; run out of available bores, get forgotten at the back of sheds or get replaced by cnc barrels. Then there is the other debate. Many believe the best combination for dd is the older small bearing crank (less inertia) combined with the newer cnc barrel. That seems to be the preferred combo for many team drivers!
In short, unless you are absolutely convinced of a cast engines pedigree or unless you want it for clubman the low risk option is to go cnc! then either go for the added reliability and newer engine with a big bearing crank or the possible small advantage of the small bearing
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